Microwave resonators are used in various sensing applications ranging from material identification and classification, gas sensing and detection, and environmental system monitoring, to name but a few. One type of resonator is a microwave planar split-ring resonator, which has a planar structure with a simple, low-cost fabrication process and operation form factor. A planar resonator allows for noncontact sensing, and easy integration and compatibility with complementary metal oxide semiconductor (CMOS) technologies. Resonators have been used in microfluidic devices for the label-free detection of biomolecules and the detection of various concentrations of a target material in solution.
A planar split-ring resonator detects the variation in a nearby medium through variations in the electric field above a substrate of the resonator. However, planar resonators generally suffer from low sensitivity and resolution since they have a low quality factor (Q factor), for example up to 300. The quality factor is a dimensionless parameter that represents energy losses in an under damped oscillator or resonator. A definition of quality factor (Q) is the ratio of the energy stored in the oscillating resonator to the energy dissipated per cycle by damping. Quality factor of a resonator also is defined by the resonance frequency divided by bandwidth of the signal. This also means for higher quality factors, the signal has smaller bandwidth.
As a result of planar resonators having a low quality factor, the distance between a sample being investigated and the resonator is typically minimized to attempt to optimize the effects of the sample on the electric field. In this regard, the resonators have a small field of view. Also, the low quality factor does not allow for small variation detection. Bringing the sample as close as possible would make the effect of the sample variation more detectable. Furthermore, the permittivity sensing resolution of passive planar resonator sensors having a low quality factor is also low, particularly when the sample or the environment through which the sample is sensed is lossy (e.g. absorbs part of the signal emitted by the sensor). A lossy sample or environment degrades the quality factor of the sensor, which in turn reduces the accuracy and resolution of the sensor.